A nitride semiconductor including characteristics such as high saturation electron speed and wide band gap can be utilized in a semiconductor device having a high withstand voltage and high output power. For example, the band gap of GaN as the nitride semiconductor is 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV), and thus GaN has high breakdown electric field intensity. Accordingly, GaN is quite promising as a material of a semiconductor device for power supply which obtains high voltage operation and high output power.
As a semiconductor device using the nitride semiconductor, there have been made numerous reports on a field effect transistor, particularly a high electron mobility transistor (HEMT). For example, among GaN-based HEMTs (GaN-HEMTs), AlGaN/GaN.HEMT using GaN as an electron transit layer and AlGaN as an electron supply layer is attracting attention. In the AlGaN/GaN.HEMT, a strain resulted from a lattice constant difference between GaN and AlGaN occurs in AlGaN. Two-dimensional electron gas (2DEG) of high concentration is obtained from piezoelectric polarization and spontaneous polarization of AlGaN caused by the strain. Accordingly, the AlGaN/GaN.HEMT is expected as a high efficiency switch element and a high-withstand-voltage electric power device for electric vehicle, or the like.    Patent Document 1: Japanese Laid-open Patent Publication No. 2012-178467
In the GaN-HEMTs, a current collapse phenomenon that electrons are trapped in an element to interfere with the flow of 2DEG and decrease output current is seen as a problem. The current collapse is regarded as occurring due to various causes, and the current collapse is considered to possibly occur due to an insulating film such as a protective film or the like covering the gate electrode. However, any technique useful for suppressing occurrence of the current collapse has not been developed at present for the protective film covering the gate electrode.